Battery systems with low cost, high energy density, safe operation and long cycling life time have been sought after as viable technologies for storing sustainable energy and have also been greatly desired to meet increasing demands of powering portable devices and electric vehicles (EVs). In this regard, advanced rechargeable batteries can help to reduce the use of fossil fuels and the emission of CO2. Recently, Mg batteries have attracted increasing attention as a promising high energy density battery technology and alternative to lithium-based batteries for grid scale energy storage, portable devices, and transportation applications.
Magnesium as an anode material inherently possesses a number of benefits. It is relatively safe to use without jeopardous dendrite formation. It is earth abundant, relatively low in cost, and has a high volumetric capacity (3832 AWL) due to the divalent nature of the Mg2+/0 redox couple. However, the lack of practical, high-performance Mg2+ electrolytes has been a primary technical hurdle to the development of practical Mg2+ batteries. Unlike Li+ salts, simple Mg2+ salts (e.g. MgCl2, Mg(ClO4)2, MgTFSI2, MgSO4 etc.) in organic solvents are not electrochemically active for Mg+ plating and stripping because of the formation of passivation films on electrode surfaces.
The current methods for preparing Mg2+ electrolytes involve the use of nucleophilic sources and/or Grignard reagents (or analogues like RMgCl and MgR2). However, these nucleophilic sources and Grignard reagents are difficult because they are highly reactive and because employing them for synthesis of Mg2+ electrolytes is complex and can result in low yields. Furthermore, their presence can limit oxidation stability of the Mg2+ electrolyte. Further still, the highly reactive chemicals can have limited compatibility with electrophiles such as high capacity cathodes (e.g., sulfur cathodes) and related electrolyte additives.
As an example of these disadvantages associated with the presence of these nucleophilic species, poor oxidation stability and undesired nucleophilicity of the resulting electrolytes is due to incomplete reactions and/or byproduct formation (i.e. MgR2 generated from RMgCl. Electrolytes using RMgCl or MgR2 are precluded from being included in high energy density Mg batteries with sulfur cathode (i.e., electrophilic) materials because of the formation of disulfide species.
Mg(BH4)2 and Mg(BPh4)2 have also been considered as active Mg2+ electrolytes. However, low oxidation stability or insufficient coulombic efficiency of these mono-component electrolytes limits their practical application. Accordingly, a need exists for improved electrolytes for Mg-based energy systems and methods for synthesizing such electrolytes.